Many silicone pressure sensitive adhesives are produced by mixing a solution of a branched silicone resin, specifically an MQ silicone resin with a polydiorganosiloxane and optionally a catalyst for condensation of the polydiorganosiloxane with the MQ silicone resin and/or a crosslinking agent for the polydiorganosiloxane.
By a ‘MQ silicone resin’ we mean a polymer comprised primarily of R3SiO1/2 and SiO4/2 units (the M and Q units, respectively) wherein R is a functional or non-functional, substituted or unsubstituted monovalent radical. The MQ silicone resin may also include a limited number of R2SiO2/2 and RSiO3/2 units, respectively referred to as D and T units. As used herein, the term “MQ silicone resin” means that, on average, no more than about 20 mole percent of the resin molecules are comprised of D and T units.
A siloxane is a compound which contains at least one Si—O bond. A polysiloxane is a compound containing several Si—O—Si— bonds forming a polymeric chain, where the repeating unit is —(Si—O)—. An organopolysiloxane is sometimes called a silicone. An organopolysiloxane contains repeating —(Si—O)— units where at least one Si atom bears at least one organic group. “Organic” means containing at least one carbon atom. An organic group is a chemical group comprising at least one carbon atom. A “silicone resin” or “resin” is a silicone comprising T and/or Q units.
MQ silicone resins are generally prepared in solution in an aromatic hydrocarbon solvent such as xylene or toluene, and are usually sold as solutions in the aromatic hydrocarbon solvent in which they were prepared. Thus silicone pressure sensitive adhesives are produced by dissolving a polydiorganosiloxane in a solution of a MQ silicone resin in an aromatic hydrocarbon solvent.
For some uses the solvent in which the MQ silicone resin was prepared is not a suitable vehicle for a pressure sensitive adhesive for that particular use. The process of the present invention allows the production of a pressure sensitive adhesive in a solvent different from the solvent in which the MQ silicone resin was prepared.
U.S. Pat. No. 5,726,256 describes producing a pressure sensitive adhesive by mixing a solution of a MQ silicone resin with a polydiorganosiloxane and an acid catalyst and reacting the MQ silicone resin with the polydiorganosiloxane. Reaction of polydiorganosiloxane with MQ silicone resin is typically called “bodying.” U.S. Pat. No. 5,861,472 describes producing a ‘bodied’ pressure sensitive adhesive from a solution of a MQ silicone resin, a polydiorganosiloxane and a base catalyst. WO2007/067332 describes a continuous method for preparing a pressure sensitive adhesive by mixing a hydroxyl-functional polydiorganosiloxane polymer, a hydroxyl-functional polyorganosiloxane resin and a solvent while heating the composition at a temperature above the vaporization point of the solvent and removing volatile species in a devolatilizing twin-screw extruder.
U.S. Pat. No. 5,324,806 describes forming a free flowing silicone powder having a primary particle size in the range of 0.1-200 nm and an aggregate size of 10 nm to 200 microns by spray drying an organic solvent dispersion of a MQ resin. U.S. Pat. No. 5,319,040 describes capping such MQ resin by reaction with an organosilicon nitrogen material, for example a silazane, before spray drying. Either type of spray dried MQ resin can be used to make heat curable organopolysiloxane compositions, such as a pressure sensitive adhesive. U.S. Pat. No. 5,357,007 describes mixing spray dried MQ resin with a fluid network mixture of an alkenyl siloxane, a silicon hydride siloxane and a hydrosilylation catalyst to form a pressure sensitive adhesive.
U.S. Pat. No. 8,017,712 describes producing a solid solventless MQ resin by feeding a MQ resin dispersed in a volatile solvent into an extruder, removing the volatile solvent and recovering the extruded solid solventless MQ resin.
A volatile compound is a compound which is easily evaporated at room temperature (20-25° C.). A volatile compound has a high vapor pressure at ordinary room temperature. A volatile compound has a low boiling point, typically less than or equal to 250° C. measured at a standard atmospheric pressure.